Immune nertropenia may be idiopathic, drug-related or associated with other immune diseases. Much remains to be learned with regard to autoantibody identity and specificities, the mechanisms of their effects, and the reasons for occurrence of autoantibodies. Hypotheses underlying this proposal are 1) that the pathophysiology of immune neutropenia relates to the consequences of antibody binding and that the antigen to which antibody binds is an important determinant of these consequences, and 2) that the characteristics of antibodies and antigens involved in immune neutropenia relate to the underlying cause of immune dysregulation. Four interrelated objectives will test these hypotheses: a) to identify patients with immune neutropenia, using assays which measure the presence and effects of antibodies in the patients' serum which will bind to processed normal granulocytes and derived human myeloid cell lines (e.g. HL-60). Assays to be used measure IgG binding, IgM binding, complement fixation, and complement fragment generation, relying chiefly on radioiodinated monoclonal antibodies; b) to identify and characterize antigens to which autoantibodies are directed. This will be accomplished by using autoantibodies (e.g. granulocyte eluates) and granulocyte (or HL-60) membrane components in radioimmunoprecipitation and immunoblotting procedures; c) to explore the consequences of antibody binding and complement activation. This will be done using the in vitro assays listed above along with assays of cytotoxicity and granulocyte function; d) to correlate antibody specificity and consequences of antibody binding to clinical disease. Using standard statistical techniques results will be analyzed for significance to determine validity of original hypotheses. The definition of antigen specificities, antibody characteristics, and consequences of antibody binding will constitute the biochemical characterization of syndromes of immune neutropenia and will allow appropriate classifications and analysis of development, course, and response to therapy in various syndromes so that future therapies can be formulated on a rational basis. In addition this proposal for basic immunohematology research has broad relevance to questions about neutrophil maturation and function, and the regulation of antibody production, complement activation and degradation, and other aspects of humoral and cell-mediated immune phenomena.